Frequency converters typically have a DC voltage intermediate circuit which stores DC voltage for use for the inverter part. The inverter of a frequency converter is typically used for producing controlled alternating voltage to a load from the voltage of the intermediate circuit. The DC voltage to the intermediate circuit is rectified typically from AC mains voltage by using a rectifier bridge.
The DC voltage intermediate circuit or DC bus contains one or multiple capacitors for storing and smoothing the voltage. The capacitance of the intermediate circuit capacitors is large and when the frequency converter is taken into use, these capacitors have to be charged before any control operations can be carried out.
The capacitors of the intermediate voltage circuit are charged from the supplying mains voltage. The charging current has to be limited to a suitable level since otherwise the large current would harm the electrical components of the rectifier or cause erroneous operations of the protective components, such as fuses and like. The current limitation is typically carried out either by using a charging resistor or by controlling the controllable switches of the rectifier.
One type of rectifier commonly used also in frequency converters is a half-controlled rectifier bridge. The half-controlled bridge consists of multiple series connections of thyristors and diodes, and usually the thyristors are the upper components and thus their cathodes are connected to the positive rail of the DC intermediate circuit.
Once the intermediate circuit capacitor has been charged to a voltage that is near the value of the rectified mains voltage, the charging phase is over and the thyristors in the rectifier are usually operated as diodes. This means that the thyristors are controlled to a conducting state as soon as the anode to cathode voltage is positive, and the thyristor can be triggered to a conducting state. This operation of thyristors in diode mode, i.e. at full phase angle, ensures maximal voltage to the DC intermediate circuit.
In the prior known solutions the control of thyristors requires synchronization to the supplying network. The synchronization can be carried out with measurements of the voltages of the supplying network together with a phase lock circuit, which keeps the control of the thyristors in phase also in connection with dynamic changes in the network. The operation of the thyristors in the diode mode should be ensured also in special cases, such as during earth faults and blackouts.
One of the problems relating to known solutions thus relates to the need for synchronization of the rectifier bridge with the supply network for controlling the thyristors to the diode mode.